In advanced slagging combustion systems for the combustion of particulate carbonaceous materials, such as coal, introduced with a carrier fluid which may be liquid or gaseous, it is important that ignition be achieved as quickly as possible and that the flame front be maintained at or close to the point of fuel introduction. If not, there will be a delay in ignition and, because the residence time in the slagging combustor is in the order of a few-hundred milliseconds, a greater chance exists that combustion instabilities may arise, and/or that fuel particles may exit the combustion chamber before the carbon content of the particles is converted to gaseous products of combustion. In addition, if the flame front is too far away from the point of injection, the flame tends to be unstable.
In the slagging combustion system described herein, a nozzle assembly projects into the combustion chamber. Active combustion takes place at or close to the orifices of the nozzle, i.e., atomizer or pintle. To avoid agglutination and/or partial carburization of the powdered coal, with consequent clogging of the nozzle assembly, the injector assembly normally is fluid-cooled. Fluid cooling the injector increases its durability and reliability; but such cooling also tends to cool the mixture of oxidizer, fuel and combustion products surrounding the injector. This adversely affects combustion. The problem is aggravated in the use of coal-water slurries, where a large amount of water is injected into the combustor and requires vaporization, but is also significant when particulate coal is fluidized and introduced by means of a carrier gas.
In this class of high-power-density combustion systems, the fuel injector is immersed in a mixture of oxidizer, fuel and combustion products at temperatures of the order of 2000 to 3800 or more degrees F. Yet, the injector per se must operate at temperatures low enough for fuel to flow through the injector passageways without significant agglomeration, carburization or plugging of these passageways. At the same time, for good flame stability and consistently low-NO.sub.x combustion, the combustion mixture adjacent the injection assembly ought to be kept at a more-or-less uniform operating temperature. Thus the primary object of my invention is to keep the injector relatively cool, while preventing it from significantly inhibiting or delaying combustion in the surrounding space.
The present invention meets the foregoing objectives by providing a barrier for minimizing transfer of thermal energy to the injector from the surrounding mixture of fuel and gas. It prevents the injector from cooling the adjacent gases, and protects and injector from potentially-damaging thermal flux.